1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to an improved method and medical device for measuring electrical bio-impedance signals:
2. Description of the Prior Art
Electrical bio-impedance signals has been found to be an useful tool in implantable medical devices, such as cardiac pacemakers. For example, electrical bio-impedance signals can be used to determine the amount of air being inhaled by a patient to assess the patient's need for cardiac output in order to provide a proper pacing pulse rate to the patient since measuring the impedance of the chest cavity has been found to provide a good indication of the amount of air being inhaled by a patient. One example of this is described in U.S. Pat. No. 5,197,467. Furthermore, electrical bio-impedance has been found to be an effective measure for identifying changes of many different conditions in the body of a patient, such as incipient pulmonary edema and the progression of pulmonary edema due to CHF. For example, the accumulation of fluids in the lung-region associated with pulmonary edema affects the thoracic impedance, since the resistivity of the lung changes in accordance with a change of the ratio of fluid to air. As indicated in U.S. Pat. No. 5,957,861, it is possible to detect pulmonary edema at an early stage by means of transthoracic impedance measured by an implanted device such as a pacemaker.
In addition to the thoracic impedance, the cardiogenic impedance, which is defined as the impedance or resistance variation that origins from cardiac contractions measured by electrodes inside or on the surface of the body, can be used for identifying changes different conditions in the heart of a patient. The cardiogenic impedance variation correlates to the volume changes of the heart chambers, which can be used as an indication of the dynamic blood filling. Hence, changes of parameters such as left ventricular ejection time (LVET) can be detected by monitoring or detecting changes of the cardiogenic impedance, see, for example U.S. Pat. No. 6,511,438.
Accordingly, a reliable and accurate method for measuring or detecting electrical bio-impedances, such as the intrathoracic impedance or the cardiogenic impedance, i.e. the cardiac component of an impedance signal measured over the heart, in an implantable medical device would be of a great value.
Conventionally, when generating multi-phasic impedance measurement current pulses, a positive and a negative current of equal amplitude and width has been used in order to only make use of fully balanced signals, i.e. integrating over the complete current pulse should ideally give a zero area. It is believed that a multi-phasic, for example, a biphasic excitation pulse offers the advantages over a monophasic pulse that the peak amplitude of the excitation pulse is minimized given the overall energy content of the pulse, electrode polarization is cancelled, and DC current is balanced to avoid long-term lead metal-ion oxidation, see, for example, U.S. Pat. Nos. 6,269,264, and 6,104,949. However, since the pulse generator of a conventional implantable medical device, such as a pacemaker, typically operates around 0.50.9 V with the case or housing of the device as the negative supply, i.e. 0 V, and the battery (about −2.8 V) as the positive supply, the negative voltage headroom is significantly lower than the positive headroom. The current pulse amplitude is hence limited by the negative voltage headroom (negative available voltage) and the complete potential operating range of the pulse generator is not utilized. This may, in turn, entail to an impaired signal-to-noise ratio of the measured impedance signal.
Accordingly, there is a need of an improved method and medical device that are able to obtain reliable and accurate electrical bio-impedance signals.